Study of the transmembrane potentials of canine Purkinje fibers is proposed. It is planned to determine the experimental conditions which either facilitate or prevent the occurrence of "slow responses" in isolated Purkinje fibers exposed to reduced extracellular pH. Experiments will be done in which the effects of variation in the extracellular concentration of K ion, Na ion and Ca ions on the onset of slow responses will be studied. In addition the influence of catecholamines and adrenergic blocking drugs will be evaluated. These experiments are of interest because of the possible role slow responses play in the cardiac arrhythmias which arise during myocardial ischemia.